Around 1870 the German physicist Ernst Abbe of the University of Jena established the theory of optical imaging and deduced the resolution limit of lenses. Before Abbe, making a good lens was a matter of trial and error. Abbe's theory enabled him and his collaborators, the optician Carl Zeiss and the entrepreneur Otto Schott, to create the modem optics industry. Carl Zeiss Jena is still a household name more than a hundred years later.
All conventional lenses have limited resolution. Even with the strongest microscope it is not possible to see atoms, molecules or nanostructures; for this electron or atomic-force microscopy is needed. The wavelength of light sets the resolution limit, half a micrometer for visible light.
In 2000, Sir John Pendry of Imperial College London published [Pendry J B 2000 Phys. Rev. Lett. 85 3966] a remarkable theoretical result: a lens made of a negatively refracting material (i.e. a lens that bends light in the opposite direction from a positively refracting material) is perfect in theory—that is a flat lens made of negatively-refracting material [Veselago V G 1968 Sov. Phys.—Usp. 10 509] can, in principle, image light with unlimited resolution, beyond Abbe's limit. Since then, negative refraction has been believed to be key to the achievement of perfect imaging.
Negative refraction occurs in materials with both negative dielectric c and p; it can also be realized in other cases, for example in photonic crystals [P. V. Parimi et al., Nature 426, 404 (2003)], but perfect imaging with negative refraction requires negative ∈ and μ. Negative refraction has been subject to considerable debate (see J. R. Minkel, Phys. Rev. Focus 9, 23 (2002) for a review) but the consensus of the majority of physicists working in this area is that negative refraction is real. In particular, experiments [J. Yao et al. Science 321, 930 (2008); J. Valentine et al., Nature 455, 376 (2008)] demonstrated a negative Snell's law of refraction for infrared light.
The quest for the perfect lens thus initiated and inspired the rise of research on metamaterials, believed to be capable of negative refraction [Veselago V G 1968 Soy. Phys.—Usp. 10 509] (an optical property not readily found in natural materials).
Metamaterials may be engineered to exhibit negative refraction (see Smith D R, Pendry J B and Wiltshire M C K 2004 Science 305 788 and Soukoulis C M, Linden S and Wegener M 2007 Science 315 47), but in such cases they tend to be absorptive and narrowband, for fundamental reasons. In particular, Stockman [Stockman M I 2007 Phys. Rev. Lett. 98 177404] showed that negative refraction is always restricted to a small bandwidth and can only occur in dissipative materials. Thus, in practice, the fact that negatively refracting materials absorb light quickly thoroughly spoils their imaging potential. In addition, the super-resolution is easily lost when the lens becomes comparable in thickness to the wavelength; only “poor-man's lenses” that are substantially thinner than the wavelength have shown sub-wavelength imaging beyond the diffraction limit [N. Fang et al, Science 308, 534 (2005)].
The resolution limit of lenses limits the microchip technology needed for making ever faster computers. Chipmakers photograph the structures of billions of tiny transistors on silicon chips. To meet the insatiable appetite for more and more transistors that need to be smaller and smaller, the resolution limit of lenses forces chipmakers to use light with ever shorter wavelength, which gets increasingly difficult. An alternative imaging method which allows improved resolution is therefore required.
Suggested alternatives to negatively refracting materials include hyperlenses [Z. Jacob, L. V. Alekseyev, and E. Narimanov, Opt. Express 14, 8247 (2006)] that rely on materials with indefinite metric. These lenses are made from anisotropic materials where one of the eigenvalues of c is negative; these materials thus implement a hyperbolic geometry (hence the name hyperlens). Hyperlenses are able to funnel out light from nearfields without losing sub-wavelength detail, but their resolution is determined by their geometric dimensions, and is thus not unlimited.